The image sensor can easily be made for example in Complementary Metal Oxide Semiconductor (CMOS) type technology. Such CMOS image sensors are used, for example, for making photographic devices that can be fitted to small sized portable instruments, such as wristwatches. The electric power consumption of each electronic component has to be reduced in these instruments, which are powered by a battery or an accumulator. Consequently, the image sensor has to be made to consume a small amount of electric energy during image taking or processing operations.
Owing to current integration technology, this type of image sensor includes, on the same chip, a photosensitive cell formed of a set of pixels, and a processing component for performing image taking operations and reading the information picked up by the photosensitive cell. The pixels are typically organised in the form of a matrix arranged in rows and columns. The matrix occupies most of the sensor surface. In order to read a particular pixel of the matrix, the corresponding row and column are addressed. The sensor therefore conventionally includes a row addressing circuit coupled to the matrix rows and an output bus coupled to the matrix columns, both controlled by a control circuit.
Photodetector elements of the cell pixels can be formed of p-n junction capacitors of a semiconductor substrate for picking up photons. These junction capacitors are commonly called photodiodes, which have the advantage of being compatible with standard CMOS manufacturing processes.
In normal operation, each photodiode is inversely polarised at a given voltage, for example between 0 and 2 V. The photons picked up by the photodiode discharge one photodiode capacitor generating electron-hole pairs. These electron-hole pairs are collected by opposite electrodes of the capacitor and consequently reduce the voltage gap at the capacitor terminal within a determined dynamic voltage range of the sensor. This dynamic voltage range of the sensor is less than the photodiode polarisation voltage, for example equal to 1.5 V, but this condition is not, however, limiting.
Each pixel of the matrix can possess a structure in conformity with the illustration of FIG. 1, equivalent to the structure of FIG. 2B of EP Patent No. 1 128 661 by the same Applicant, of which the description relating to FIG. 2B and to the method for obtaining an image is incorporated herein by reference. This pixel 1 comprises a element, such as an inversely polarised photodiode PD, storage means, such as a capacitor C1 and five transistors M1 to M5, for example of the n-MOS type. The photodiode collects the electrons photo-generated during an integration or exposure period, whereas the storage means stores the voltage value present at the terminals of photodiode PD during a sampling phase.
Transistor M1 is connected in series with the photodiode between a high electric supply terminal VDD and a low electric supply terminal VSS of a voltage source that is not shown. According to the prior art, this transistor M1, which is controlled by a initialization signal TI across its gate terminal, initializes or resets photodiode PD to a determined voltage before each integration or exposure period.
Transistor M2 connects capacitor C1 to the connection node between transistor M1 and photodiode PD. This transistor M2, which is controlled by a sampling signal SH, samples the charge accumulated by photodiode PD and stores the signal thereby sampled in the capacitor. This transistor M2 also insulates or uncouples photodiode PD and capacitor C1.
Transistor M3 is connected in series with capacitor C1 between the two electric supply terminals VDD and VSS. According to the prior art, this transistor M3, which is controlled by a reset or initialization signal RST, initializes the capacitor to a determined voltage.
Transistor M4 is a source follower transistor, whose gate terminal is connected to the connection node between the source terminal of transistor M3 and capacitor C1, and the drain terminal is connected to high electric supply terminal VDD. Transistor M4 is arranged in series with transistor M5, which is a row selection transistor. Transistor M5, which is controlled by a row selection signal RSEL, transfers, during the read process, the voltage from transistor M4 onto an output bus common to all the pixels in one column.
With reference to FIG. 2, the conventional method of obtaining an image using an image sensor is described, with the structure of each pixel being shown in FIG. 1. FIG. 2 thus shows a temporal diagram of the evolution of control signals TI, SH, RST and RSEL for operating the pixel structure of FIG. 1, and it shows schematically the evolution of voltage VPD of photodiode PD and the evolution of voltage V1 across capacitor C1.
During a first initialization or reset phase, the first and second initialization signals TI and RST are both brought to a high positive voltage close to VDD. In this manner, photodiode PD and capacitor C1 of each pixel are both reset to a determined reset voltage. Sampling signal SH is at a low level such that transistor M2 is not conductive, which enables photodiode PD and capacitor C1 to be uncoupled. Likewise the row selection signal RSEL is at a low level so that row selection transistor M5 is not conductive. The resulting voltages VPD and V1 on photodiode PD and capacitor C1 are then at levels substantially equal to the determined initialization voltage.
At the end of the initialization phase at time t1, the first initialization signal TI passes to a low level making transistor M1 non conductive, which starts the exposure or integration period of photodiodes PD of the image sensor. Via the effect of illumination, photodiodes PD start to discharge proportionally to the quantity of light that each of them receives as shown by the evolution of voltage VPD between t1 and t3. Initialization signal RST is kept in the high state to keep capacitor C1 at a constant voltage level close to VDD.
After a determined exposure period at time t2, the second initialization signal RST passes to a low level, thus releasing the memory node in capacitor C1. Sampling signal SH then briefly passes to a high level making transistor M2 conductive. This enables the voltage value present on photodiode PD to be sampled and stored in capacitor C1. Voltage V1 at the terminals of capacitor C1 thus evolves as shown in FIG. 2. At time t3, the sensor exposure period ends. The capacitor of each pixel keeps the photodiode voltage value in its memory as a function of the illumination thereof as soon as transistor M2 is made non conductive.
Once the voltage value has been stored in capacitor C1, the first initialization signal TI is again brought to a level such that each photodiode is again initialized to a voltage substantially equal to the initialization voltage close to VDD. The sampled voltage stored in the capacitor of each pixel of the sensor is thus not disturbed by the phenomenon of charge carrier diffusion, so that the voltage present in this capacitor remains constant. A phase of reading the voltage value stored in the capacitor occurs at time t4 for each row of pixels of the sensor in succession.
In a determined exposure period when an image is being taken, the higher the number of photons picked up by each photodiode, the more quickly the photodiode capacitor is discharged. Generally, in the case of strong illumination of the photodiode, the photodiode capacitor is discharged quickly to a minimum voltage value that is a function of the dynamic voltage range of the sensor, which is not illustrated in FIG. 2. Conversely, in the even of low or average (moderate) illumination of the photodiode, the voltage at the photodiode capacitor terminals gives material information as to the image picked up. In order to obtain material information as to the image picked up in strong and weak illumination, at the end of a defined exposure period, the sensor must comprise means for increasing its dynamic voltage range.
In this regard, various methods for increasing the dynamic range of an image sensor have already been proposed. One of the methods consists for example of using digital processing to merge two images taken at different exposure times. As this requires storing an image in the memory, this uses a lot of space in the circuit. This cannot therefore be applied to an image sensor able to be fitted to an instrument of small volume. It is also possible to add several electronic components in the pixels of the photosensitive cell of the image sensor in order to increase the dynamic range. However in that case, this considerably reduces the light collecting surface ratio on the pixel surface, which is a drawback.
One can also cite WO Patent No. 2004/064386, which discloses an image sensor with transfer function control for extending the dynamic range of the sensor. An inversely polarised photodiode is connected in series to an MOS type initialization or reset transistor between two terminals of a power source. This MOS transistor initially charges the photodiode to a first determined initialization voltage close to the high voltage value of the power source. The MOS transistor is then disconnected in order to start a first long exposure period of the photodiode in parallel with a charge storage capacitor. As a function of the level of illumination of the photodiode, the capacitor is discharged more or less quickly during the first exposure period. The maximum that the capacitor can be discharged is to a minimum voltage value defined by the dynamic range of the sensor if the photodiode is strongly illuminated.
A second photodiode initialization operation is carried out by the MOS transistor at a lower level than the first initialization voltage. In the case of low illumination of the photodiode, this second initialization operation has no influence on the photodiode voltage level. Conversely, in the case of a strong illumination of the photodiode, the photodiode voltage level is initialized to a second initialization voltage lower than the first initialization voltage. The MOS transistor is then disconnected again to start a second short exposure period of the photodiode.
During the various exposure periods of the photodiode, an operation of reading the charge accumulated in the capacitor is carried out via an assembly of follower transistors. Owing to at least two exposure periods of the photodiode, it is thus possible to obtain material information for a photodiode that is weakly, averagely (moderately) or strongly illuminated, which has the effect of extending the dynamic voltage range of the image sensor.
One drawback of the solution proposed in WO 2004/064386 is that the charge storage capacitor is directly placed in parallel with the photodiode. Consequently, at every photodiode reset operation, the capacitor is also initialized or reset to the photodiode voltage level. Several operations to read the voltage value in the capacitor must thus be performed, which complicates the processing of the information provided by each pixel.
It should also be noted that a single initialization transistor is used to initialize the photodiode before each exposure period. Consequently, it is necessary to adjust the gate voltage of the MOS transistor to different voltage levels during the various photodiode initialization operations, which can complicate the manufacture of the image sensor.